Driving with inverters with low switching losses

ABSTRACT

The invention relates to converters (inverters, pulse or frequency converters) and to the driving of “magnetically active” operating means. An effective switching frequency of the converter is not to be reduced, but nevertheless a reduction in cooling requirements is to be achieved A noise level produced in the operating means is to be kept low as well. The invention proposes, for this purpose, a circuit arrangement for feeding the operating means (electrically operated machine (M)) having at least one winding (L 1,  L 2,  L 3 ), which circuit arrangement, in at least one first winding phase (S 1 ), comprises a first branch (Z 1 ) of a frequency converter (WR 1 ) adapted for and operable at a switching frequency of not higher than 5 kHz for outputting a main alternating current generated at said switching frequency and having a substantially lower operating frequency (f 1 ) to a winding (L 1 ). A second branch (z 1 ) of another frequency converter (WR 2 ) is adapted for and operable at a second switching frequency of more than 5 kHz for outputting a supplementary alternating current generated at said switching frequency to the same winding (L 1 ). In the at least one winding (L 1 ), the two alternating currents (i A (t); i B (t)) of the two branches (Z 1,  z 1 ) are superimposed to form a sum current, namely during parallel operation of the first and second branches (Z 1,  z 1 ) of the two non-identical inverters.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Stage Application of InternationalApplication No. PCT/EP2007/055983, filed Jun. 15, 2007, which claims thebenefit of German Patent Application No. DE 10 2006 027 716.3, filed onJun. 15, 2006, the disclosure of which is herein incorporated byreference in its entirety.

BACKGROUND OF THE INVENTION

The invention relates to converters (inverters, pulse or frequencyconverters) and to the driving of, for example, machines and isimplemented in form of a device (circuit arrangement) or a method forfeeding such a magnetically active operating means in the sense of atransformer, a motor/generator (more general: a machine). A drivingarrangement is concerned with the regulation or control of said circuitarrangement.

High demands are made to electrically operated driving means as anexample of “magnetically active”, electrically operable operating means,on the one hand, regarding the noise behavior and, on the other hand,regarding the dynamics of regulation provided by frequency convertersand servo converters as inverters. These converters provide analternating current generated by a high switching frequency which, inmachines of high power of more than 10 kW to 30 kW and, in particular,in draft-ventilated motors, results in the occurrence of disturbingnoise levels in the motors caused by feeding the converters at a too lowswitching frequency.

In order to avoid or at least reduce said noise levels, the pulsefrequencies (=switching frequencies) of the inverters must be increased,which also increases the power loss in said inverters, if they arerequired to switch high load currents of the machines. This isimplemented at a “high switching frequency” which may be higher thane.g. 8 kHz. If the inverters (converters) are switched at thisfrequency, the noise levels may be kept within acceptable limits at thecost of an increased power loss in the converters which consist of aplurality of branches each, wherein, mostly in three-phase machineshaving three magnetically active and electrically fed windings, aconverter having three phases and adapted to said windings may also beused.

If highly dynamic driving means are used, e.g. linear motors or motorswhich are required to synchronize with a high speed of a belt or webmoving past it, the high pulse frequency is not only useful for reducingthe noise level, but is also required to enable realization of a fastcurrent regulation of said highly dynamic driving means (asrepresentatives of magnetically active operating means).

If this first or the second or the common goal is to be achieved today,high switching frequencies are inevitable in inverters which then, evenat high output currents, lead to high switching frequencies and highcooling requirements. These high requirements are caused by the factthat the power semiconductors fulfilling the high switching frequency inthe respective branches of the inverter are required to switch the highgenerator or motor load current of the machine at said high frequency sothat the power semiconductors, transistors or IGBTs, have to beappropriately dimensioned and mounted accordingly in the cooling or fordissipating the produced heat.

BRIEF SUMMARY OF THE INVENTION

It is the object of the invention not to reduce an effective switchingfrequency of a converter or inverter for a “magnetically active”operating means, for example an electric machine, but nevertheless toachieve a reduction in cooling requirements, in particular, also indriving the converter. Reducing the cooling requirements is to beunderstood as involving three aspects, firstly reducing the productionof heat, then reducing the necessary dissipation of heat, for example bythe shape of the cooling elements, and thirdly reducing the arearequired for dissipation. In other words, “the cooling requirements” forproperly driven and operated power semiconductors in the inverter are tobe reduced. If this can be achieved while not or non-essentially havingto reduce the switching frequency in order to keep the noise levelsgenerated in the operating means low as well, a complex problem to besolved arises.

This problem is solved by a circuit arrangement having two frequencyconverters (=inverters) operating in different ways and having acompletely different structure, even in a physical sense (claim 1). Thedriving thereof in the sense of regulation/control is defined in form ofa driving arrangement (claim 22). Said driving arrangement comprises atleast one regulating means. A comparable driving arrangement (claim 25)comprises two current regulators which are capable of driving andoperating the circuit arrangement (claim 1). A combination of thecircuit arrangement (claim 1) and a driving arrangement (claims 22, 25)is defined in claim 30. An associated operating method concerned withthe predetermination of switching frequencies is the subject matter ofclaim 35. An operating method for predetermining the switchingfrequencies of the two physically different branches according to claim1 is defined in claim 20.

This overview is intended to show which subject matters are defined inthe claims.

The distinction over the prior art resides to a lesser degree in theused switching concept of the converters, but rather in the employedpower semiconductors and in the used switching frequency and thus alsoin a cooling procedure required for any frequency converter.

When a person skilled in the art speaks of a frequency converter, inbrief converter, this converter comprises at least one, two or threephases which are each defined by a branch of at least two semiconductors(claim 6). An inverter for three-phase machines which are used in mostcases (claim 32 or claim 9) comprises three winding phases or threephases (claim 3). Similarly, a transformer having three—or more—windingsmay be operated as an electrical operating means having magnetic action.The mention of two winding phases or three winding phases (claims 2 and3) is not final, further winding phases may also be added, i.e. four andmore winding phases, wherein each winding phase comprises a first branchand a second branch, each of said branches having a different thresholdoperating frequency.

Claim 1 defines at least one first branch of a first frequencyconverter, which is not operable at more than 5 kHz and not adapted forthis purpose, and a second branch of another frequency converter, whichis operable and exactly adapted for operation above this frequencythreshold, in particular, far above this threshold (claim 1, claim 8,claim or claim 16).

If a number of pairs of this one pair of branches of first/secondbranches is considered in combination (claim 2, claim 3), a multi-phasefrequency converter is obtained (claim 4, claim 5), one being operablebelow said switching threshold only and one being operable above saidfrequency threshold.

The structure of at least one winding phase, as defined in claim 1 orclaim 35, is the minimum requirement for outputting a one-phasealternating current for a one-phase load, for example a one-phasemachine, having a nominal frequency. The output alternating current iscapable of fulfilling at least said nominal frequency (claim 36).

Due to the driving, the actual frequency of the winding phase may alsobe below or above said nominal frequency, depending on the operatingpoint of the machine. In most cases, the nominal frequency is below 400Hz. It will not reach the threshold switching frequency of the firstbranch of the first frequency converter (will remain substantiallylower) and will never reach that of the second branch of the otherfrequency converter. A factor of at least 10 is between the operatingfrequency (in the sense of the nominal frequency) and the upperthreshold switching frequency of the first branch and the firstconverter, respectively (claims 1, 20). This also applies to assessing alower threshold frequency of the slower switching branch to bepredetermined (claim 29), it necessary, preferably more than 500 Hz, inrelation to an average switching frequency over one period of thefundamental wave of the main current.

The upper threshold frequency defines the physical structure andstructural configuration of an actual inverter (claims 1, 2 and 3). Acorresponding operating method (claim 20), however, may show that thesefrequencies actually occur, especially during operation claim 20 insofardefines the actual proceedings in an electrically operated operatingmeans having at least one winding and being, for example, a machinehaving preferably three windings in the stator and/or rotor.

If the operating method (claim 20) is based on the thresholdfrequencies, i.e. an upper threshold frequency of 5 kHz, which is notexceeded by the first branch (as a representative of a first frequencyconverter), and a higher switching frequency of the second branch (as arepresentative of the other frequency converter), said frequencythresholds are not of the kind occurring every time the inverters(frequency converters) operate. On the other hand, however, they definean upper threshold for the first branch—in the sense of a thresholdfrequency. There is no upper threshold predetermined for the secondbranch in the operating method, wherein the second branch is capable ofswitching at higher frequencies, but is not required to switchpermanently at said higher frequencies. Thus, a plurality of frequenciesoccurs during operation—depending on the operating point and the load.In the first branch, said frequencies never exceed the first frequencythreshold. In the second branch, they may be above, even far above, saidfirst frequency threshold; however, also switching frequencies belowsaid threshold may occur. In this connection, a very careful distinctionshould be made between actually occurring frequencies and functionallypredetermined, claimed frequency thresholds in relation to the mainalternating current and the supplementary alternating current which,when superimposed, together form the sum alternating current.

Supplementation of the one-phase consideration towards a two-phaseconsideration (claim 2, claim 4, claim 5) or towards a three-phaseconsideration (claim 3, claim 4, claim 5) is possible, whereby discreteinverters are defined, wherein the first branches of the one operatingfrequency are associated with the one inverter and the second branchesof the other frequency are associated with the other non-identicalinverter, respectively. In a three-winding phase arrangement, twodiscrete inverters for two different threshold operating frequencies areobtained which is the technical definition of the “non-identical”frequency converters. Accordingly, this also applies to the operatingmethod (claims 20, 35).

If lower threshold frequencies are defined, for example, for the slowerswitching branch (claim 21a), it is useful to predetermine averagefrequencies, conveniently based on the fundamental wave of the main orsum current. This also applies to a lower threshold frequency of thefaster switching branch (claim 21b).

Without modifying the internal structure of said two non-identicalinverters (frequency converters), the output currents of the respectivecorresponding output terminals may be coupled to each other via arespective inductor, having a tap, outputting the sum current (thesuperimposed current) of the different operating frequencies of the twobranches interconnected during parallel operation (claim 7, claim 19).The inductor prevents the currents of the one branch from flowing intothe other branch, rather both currents of both branches are to besupplied, via the (magnetic) coupling, to the winding, for example ofthe motor, in which they are superimposed (claim 18).

Two branches each, operating in parallel for forming a winding phase,sum the currents generated by them, on the one hand, the mainalternating current of the low switching frequency converter and, on theother hand, the supplementary alternating current of the frequencyconverter operating at a higher frequency, so that the frequencies aresuperimposed in the sum current provided by a branch of the onefrequency converter and a branch of the other frequency converter. Thenoise level is reduced. The thermal stress of the converter providingthe main current is decreased.

In the winding of, for example, a connected machine, a sum alternatingcurrent having a ripple of high frequency is produced by driving the twobranches provided by the second frequency converter (claim 12). Themachine is thus minimized with respect to its noise level and is capableof operating in a highly dynamic manner, wherein, on the other hand, thehigh load currents have to be switched at the lower switching frequencyonly so that a correspondingly lower power loss arises in this mainfrequency converter.

Interconnection of the different frequency converters, or at least onebranch each of said two non-identical frequency converters,respectively, provides for superimposition of said different currentsand enables superimposition of both alternating currents in therespective winding of the machine such that a parallel operation isobtained which is capable of combining low power loss with low noisedevelopment (claim 13).

The two frequencies of the two branches of the two different frequencyconverters may differ considerably from each other, for example, may beabove a factor of 2 (claim 8, claim 15), in particular, in the rangebetween seven-fold and nine-fold frequency of the “slowly” operatingbranch of the frequency converter (claim 8). The thus operated operatingmeans (Betriebsmittel) having a winding, or the suitability and designof the respective frequency converter for operating, for example, amachine is above 10 kW, in particular, above 30 kW, which functionallydefines the size and rated power of the switching power semiconductorsin the branches and the electrical devices associated therewith (claim9).

In other words, the definition of the design and operability of arespective branch of a frequency converter and of a frequency converterformed when multiplying the branches, e.g. three branches, is to beunderstood such that “design” refers to the components and therespective structure of the branches, i.e. suitable for current loadingand for dissipating heat, and “operability” refers to the driving notimplementing frequencies above or below said threshold.

By this, a person skilled in the art understands a specific kind offrequency converters, on the one hand, the kind which is incapable ofoperating at pulse frequencies of more than 5 kHz and which is adaptedfor generating an alternating current high in effective value, whichaccepts the substantial load portions of, for example, the motor above10 kW. On the other hand, the second frequency converter is designed foroperation at a substantially higher frequency and is adapted foraccepting no more than lower load fractions by providing thesupplementary alternating current.

The relationship of said two loads and the load distribution is within arange of more than 5, in particular, more than 10, as measured by therespective effective values of the output currents. In an example, itmay be said that the inverter operating at a low frequency providescurrents of up to 200 A and the supplementary alternating current is inan order of magnitude of 20 A to 30 A.

A more precise distinction of the two branches, the first branch for thelower frequency and the second branch for the higher frequency, and thetwo inverters or converters having multiple first branches and multiplesecond branches, respectively, may also be provided (claim 10, claim11).

The highest threshold frequency of the slower switching frequencyconverter may also be brought down to smaller upper frequencythresholds, such as 4 kHz down to a maximum of 2 kHz. The first branchis then more limited in its switching frequency. Accordingly, the secondbranch is then able to switch at even higher frequencies (claim 11) ofmore than 5 kHz up to 16 kHz. Due to the combination of the first andsecond branches (claims 10 and 11 together), all combinations mentionedherein are to be considered as disclosed, i.e., for example, not morethan 4 kHz for the first branch and more than 15 kHz for the secondbranch.

In a relationship of frequencies of a factor of 4, i.e. 4 kHz and 16kHz, an integer multiple is obtained which may also be extended to otherfactors (claim 15). All of which again with the proviso that the definedcircuit arrangement is of the kind having the design and ability tooperate at said frequencies (second branch) or which is not operableabove the said smaller frequencies (claim 10).

In a further specific embodiment, the factor of the said frequency issubstantially a factor of 2 (claim 16).

Interconnection of the two branches (phases of the one and the otherfrequency converters) is implemented by means of a, in particular,centrally tapped inductor (claim 7). A “winding phase” in the claimedsense is created, which is able to replace an output phase of a normalconverter by means of the advantages of the invention. The inductor maybe configured as a double winding applied to a common core, whichdecouples the two switching outputs of the first and second branchesfrom each other and which collectively supplies the two output currentsof said two branches to the motor winding. The motor winding foralternating current machines is represented by a leakage inductance anda main inductance in alternating current machines.

The current ripple generated by the “slow” frequency converter, whichcurrent ripple has the low frequency and is thus relatively large anddistinct, is compensated by the supplementary alternating current. Onlythe current ripple having the high frequency still occurs in the motor.Supplementation is implemented such that an at least smoother profile ofthe actual value of the output current is obtained, wherein compensationdoes not have to be complete, but may be implemented in substantialparts (claim 12). The current ripple (the alternating portion on thefundamental wave of the output alternating current) indicates theharmonic wave portion effective in the machine, wherein the lowerharmonic waves of the main alternating current no longer occur therein(claim 13).

For driving the first and second branches, at least one currentregulator is used (claims 22, 25). A driving arrangement is created.However, operation without a current regulator is also possible so thata generated current ripple of the slower inverter (having the lowerfrequency) is compensated by an adapted pulse pattern of the secondbranch (of the inverter having the higher frequency) or is at leastcompensated to a large extent (claim 14).

In the at least one, preferable two current regulators (claim 25)provided for two branches forming a winding phase (claim 1), driving isprovided such that compensation of the low-frequency current ripple ispredetermined by the regulator side. The regulating difference, whichthe main load current still has, as compared to the reference variablein form of its nominal current value, is connected to the regulator ofthe faster switching branch as a nominal value (claim 22). The currentregulator thus only receives a regulating difference of the firstcurrent regulator, which regulating difference cannot be compensated bythis first regulator due to the inherent limitation of the switchingfrequency of the first branch.

Both current regulators (claims 23, 25) remain discrete, wherein eachone of which regulates its actuator in form of the first branch (feature(a) of claim 1) and the second branch (feature (b) of claim 1). Each oneof the regulators has its own nominal value, which are not identical insignal.

In a regulator for three winding phases, specific ones of saidregulators may be combined. In field orientation, a current regulatormay be provided which drives the individual phase currents, at least themain load current, after a transformation from vector space.

When controlling the current of the two branches, it is not the outputcurrent which is measured, i.e. not the sum current of both branches,but the current prior to the combination of the individual currents ofthe branches (claim 26, claim 27).

An inductor wound in the same direction comprising two winding portionswhich are applied to a core in the same sense of winding, has an inputfor the load current generated at the low frequency and an input for thesupplementary load current generated at the higher frequency (claim 18).An electrically conductive connection of the other ends of the twowindings forms an output for the superimposed current which is output asa sum current from the winding phase.

The preferably centrally tapped inductor connected in this way andhaving a tap ensures that the two branches of a respective winding phasedo not substantially interfere with each other in internal measure(claim 19).

The overall size of the inductor is not critical. The inductor ispurpose-oriented and capable of letting the height of the main outputcurrent of the winding phase pass while allowing for hardly any or onlyvery low losses. Since the currents in the two windings of the twonon-identical branches differ considerably in height, the windings maybe adapted in cross-section which is, however, not required.

A cross-section of the winding adapted to the main current may beselected, which winding is then centrally tapped to define the outputterminal of the winding phase (claim 7, claim 18 and claim 19), the sameapplies to the driving arrangement (claim 26).

The regulators of the circuit/driving arrangement may be realizedanalogously, digitally or by way of programming.

BRIEF DESCRIPTIONS OF THE FIGURES

Exemplary embodiments explain and supplement the claimed invention.Application of the invention in electrically operated operating meanshaving at least one winding (for magnetic action) is implemented, by wayof example, in a motor.

FIG. 1 shows a winding phase S1 illustrating a first branch Z1 of afirst inverter WR1 and a first branch z1 of a second inverter WR2including the associated switching elements The machine (e.g. a motor)to be supplied and having three windings is also illustrated, whereinthe two shown branches Z1, z1 define the first winding phase S1 and feedthe first winding L1 of the machine M. The two other winding phases S2,S3 are not shown but are configured accordingly, which other windingphases feed the other two windings L2, L3 of, for example, the motor M.Also in this case, respective first and second branches are provided.

FIG. 2 shows a controlling circuit arrangement which regulates thecurrent in the two branches Z1, z1 of FIG. 1. A winding phase or arespective branch of two non-identical inverters WR1, WR2, an outputinductor 21 and the output current at the output terminal W1 leading tothe motor M are shown. FIG. 2 may be interpreted in three ways if theregulation is performed in field coordinates.

FIG. 3 schematically illustrates a section of the two currents of thetwo individual branches Z1, z1 for superimposing these currents ininductor DR/21 to form a sum current.

FIG. 4 illustrates the superimposed sum current i₁ (t) resulting fromthe two partial currents of FIG. 3, i.e. the main alternating currentand the supplementary alternating current.

FIG. 5 shows a simplified diagram including two inverters, one inverterWR1 for a high current at a low switching frequency and a secondinverter WR2 for a lower current at a higher switching frequency,wherein the output currents of the three inverters each having threephases are superimposed in phase via one respective inductor 21 a, 21 b,21 c and which output currents are available at three output terminalsW1, W2 and W3 in order to be able to be supplied to a motor having acorresponding number of windings L1, L2, L3 (or to the respective inputterminals of said windings) or alternatively to a transformer.

FIG. 6 a,

FIG. 6 b each show 20 ms of an entire period of current i_(A) of FIG. 3and of superimposed current i₁(t) of FIG. 4 accurate in time and onebelow the other.

FIG. 7 a,

FIG. 7 b are respective magnified diagrams of the two currents ofprevious FIGS. 6 a, 6 b.

DETAILED DESCRIPTION OF THE INVENTION

A synopsis of FIG. 5 and FIG. 1 gives an overview over the organizationof the pulse inverters and the driving method, by means of which thesymbolically illustrated machine M, in the embodiment a motor in form ofa highly dynamic servo motor, is fed.

The machine is to be understood in the sense of a motor or generator andcomprises three windings, which are either represented by windings L1,L2 and L3 or by the terminals (input terminals) thereof in the exampleillustrated in FIG. 1. Each winding symbolically has inductance andresistance. The motor M is connected, for example, in star.

A respective winding is supplied with a current i₁ (t). The showncircuit arrangement of two inverters, i.e. an inverter for highercurrent values and with main power transmission, such as inverter WR1shown in FIG. 5, and a second inverter WR2 for lower current values, buthaving a higher switching frequency, is to be described in such a waythat initially an overview is given by means of FIG. 5 and then only onebranch of each one of the two inverters, which are interconnected inFIG. 1, is explained.

Branch Z1 is associated with inverter WR1 and branch z1 is associatedwith inverter WR2. Their output terminals XA and XB, respectively, areinterconnected via an inductor 21, also referred to as DR, and theflowing current is superimposed, wherein said inductor comprises acentre tap 20. The winding is wound onto the same core in the samedirection such that a degenerative feedback is created and the currentfrom the first branch Z1 will not flow into the second branch z1 andvice versa. The superimposed current i₁(t) flows from the centre tap 20to the output terminal W1 and is supplied to the motor at the firstwinding L1.

Accordingly, the same applies to circuit arrangements S2, S3 and thebranches of the two inverters WR1 and WR2 thereof according to FIG. 5,which circuit arrangements are, however, not illustrated separatelyherein, but are to be appreciated accordingly by a person skilled in theart.

The two inverters WR1 and WR2 may either be feed via an intermediatecircuit ZRK or are connected to an alternating current network, whereinthe alternative of a common intermediate circuit having the voltage Ugis preferred.

The three inductors 21 a, 21 b und 21 c of the three winding phases ofFIG. 5 correspond to inductor 21 of FIG. 1 so that only this inductor DRor 21 is mentioned in the following when explaining winding phase S1.

The inverter (for example, frequency converters or other convertersswitching at a higher frequency), which is composed of the two invertersWR1 and WR2 and which is “viewed” by a user from the terminal side W1,W2, W3, feeds the motor M. The one inverter WR1 is operated at a lowswitching frequency, e.g. at 2 kHz, and provides the main powertransmission to the connected motor M. The second inverter WR2 isoperated at a higher switching frequency, e.g. at 16 kHz, and ensuresthat the current ripple of 2 kHz left by the first inverter iscompensated and that only a current ripple of a higher frequency, forexample, 16 kHz, occurs in the motor.

The second inverter WR2 has a further function. This function is thehighly dynamic current regulation which cannot be implemented by thefirst inverter WR1 due to its low switching frequency of less than 5kHz, e.g. even less than 2 kHz. Dynamic current regulation isaccomplished by the speed of the current ripples of a generatorfrequency of more than 5 kHz, e.g. more than 10 kHz, up to frequenciesof more than 15 kHz.

The two branches Z1 and z1 of a respective one of the two saidnon-identical inverters switching in different frequency ranges areconnected to each other via inductor 21 (or DR), wherein a centre tap 20of the inductor 21 defines the output terminal W1 (the “terminal”) of awinding phase S1 of the total inverter WR which is formed by the twoinverters according to FIG. 5. The two branches according to FIG. 1 areconnected to a winding L1 of the motor via this inductor. In thisconstellation, only low switching losses occur at up to 2 kHz in thehigh switching alternations for power transmission. On the other hand,switching losses do occur at the high switching frequency of more than 5kHz, however, they will not become large due to the small requiredsupplementary current of this branch z1 and the inverter WR2,respectively, as shown by the component currents of FIG. 3 (bottom) orthe sum current according to FIG. 4.

Current control of this highly dynamic regulating arrangement of the twosaid inverters of FIG. 5 is implemented in accordance with FIG. 2, inwhich only one winding phase and its current regulation, respectively,is illustrated for the sake of simplicity of explanation. The connectingelement is the inductor 21 with its two windings D1, D2 and its outputW1. The explanation of winding phase S1 is to be transferred to theother winding phases S2, S3 (or further winding phases).

The highly dynamic regulation of the motor current is implemented bypredetermining a nominal value of the current i_(nominal)(t) which ispredetermined for the first current regulator V1 in FIG. 2. Saidregulator controls the first inverter WR1, at least one of its firstbranches, one of which is illustrated in FIG. 1 under Z1. This branch isalso referred to as A-branch due to the switching frequency f_(A) whichis less than 5 kHz, in specific applications less than a maximum of 3kHz, and which may be especially in the range of a maximum of 2 kHz. Dueto this frequency limitation, the first current regulator is incapableof tracking the nominal current value such that the actual value isreadjusted in a highly dynamic manner. As a result, a currentmeasurement, measuring the actual value in form of current i_(A)(t),shows a distinct regulating error which is illustrated by the shadedportion in FIG. 3, top illustration. This regulating difference isdetermined by subtraction 50 of the actually measured current i_(A)(t)having the low switching frequency and the very precisely predeterminednominal value i_(nominal)(t), which difference defines a second nominalvalue Δi_(nominal)(t) which is predetermined for the second currentregulator V2 of the second inverter providing the supplementaryalternating current i_(B)(t) and also feeding the inductor 21.

Interconnection and superimposition, respectively, of said currents inthe inductor 21 causes the regulator V1 to control and accept the mainload, wherein regulation thereof is inaccurate, and causes the secondregulator V2 to contribute the supplementary load thus ensuring accuracyof the sum current i₁(t).

Both current regulators may be configured as usual, are shownsymbolically as V1, V2 and, in most cases, also comprise integralportions for setting the regulating difference to zero in the stationarystate. Each current regulator or each inverter is per se degenerated.Each current regulator implements regulation “discretely”. Acorresponding control loop of currents i_(A) and i_(B) is provided foreach branch (first and second) or for each inverter WR1, WR2, but is notshown graphically in FIG. 2. It corresponds to a current regulatorcommon in drive technology which does not have to be explainedseparately to a person skilled in the art.

It has already been mentioned, but shall be emphasized again, that thetwo frequencies f_(A) and f_(B) of the two inverter branches Z1 and z1are not identical, in particular, differ considerably from each otherand are above or below a threshold of 5 kHz according to a simpledefinition.

Preferred embodiments of distinct variations are above 10 kHz for theinverter WR2 switching at a higher frequency and at most 4 or 3 kHz forthe inverter switching at a lower frequency; however, the deviations maybe further increased so that the inverter switching at a high frequencyis above 10 kHz, in particular, above 15 kHz, whereas the inverterswitching at a low frequency is operable at most in a range of 2 kHz andis configured accordingly.

The result of FIG. 3 shows a section of a sinusoidally increasingnominal current i_(nominal)(t), a main load current i_(A)(t) switched atthe low frequency and having a corresponding regulating difference asshown by the shaded portion. The second component current, which isadded by the regulation according to FIG. 2 and the nominal valuerouting Δi_(nominal)(t) shown therein, provides for compensation of thehigh regulating deviation and reaches a sum current i₁(t) as may beoutput at the centre tap 20 of the inductor DR/21 or 21 a for the onewinding of the motor M. This “sum current” according to FIG. 4 (thesuperimposed current) is the actual output current of this first windingphase S1 comprising two branches Z1, z1 and the associated regulationaccording to FIG. 2. It is “viewed” by a load at terminal W1.Interconnection of the inverters WR1/WR2 appears to be a powerfulinverter having a high switching frequency and low cooling requirementseven though it is capable of operating in a highly dynamic manner withregard to current regulation.

The described interconnection is able to operate in such a way that twocommercially available inverters may be used together and arecorrespondingly connected and supplemented, respectively, via inductors21 a, 21 b, 21 c regarding their nominal value routing and their outputterminals.

It is thus not necessary to design and construct new inverters, ratherexisting inverters may be employed, adapted accordingly and modifiedaccording to the embodiments described herein.

A more detailed description of a branch of an inverter may be dispensedwith here since it may be referred to the general state of the art knownto a person skilled in the art. It shall be outlined in brief that abranch of the first inverter WR1 comprises a top switching transistorTA1, a bottom switching transistor TA2 and respective relief diodes inFIG. 1. The connection of the top emitter and the bottom collector isthe output XA of the first branch Z1 outputting a current modulated byswitching frequency f_(A), which switching frequency is less than 5 kHz.The fundamental frequency of this current i_(A)(t) is clearly below thisswitching frequency, which current corresponds to the operating currentof the machine M, which may be in an order of magnitude of up to 50 kHz,or up to 400 Hz in dynamic actuating procedures, but in any case differsso distinctly from the upper frequency threshold f_(A) that thisfrequency f_(A) is in any case capable of readjusting the operatingcurrent to the operating frequency of the motor at least inapproximation, as shown in the example of FIG. 3 (top diagram).

The second branch z1 of the non-identical other inverter is connectedaccordingly, only with other types of power semiconductors TB1, TB2,which are capable of outputting a supplementary current i_(B)(t) at aswitching frequency f_(B), the ripple of which corresponds, in order ofmagnitude, to the operating frequency of this branch of this inverter.

A distinction should be drawn between the nominal frequency f₀ of themachine M or a transformer and the actual frequency f₁ of the machine ortransformer, which the respectively supplied sum current i_(A)(t) has asa fundamental wave. This actual frequency is a function of load andoperation, the nominal frequency of the machine is predetermined. Theactual frequency f₁ may be above or below the nominal frequency of themachine, however, it will not reach the range of the maximum switchingfrequency f_(A) of the first inverter (switching at a lower frequency)and much less the range of the switching frequencies f_(B) of the secondinverter (switching at a higher frequency). Thus, the frequency rangesused for explanation herein, which are sometimes also referred to ashigher or lower and larger and smaller and for which also the technicalterm “substantially higher” frequency is used, are defined and renderedclear and comprehensible for a person skilled in the art.

When starting from a description of an inverter which is operable andadapted for operation at switching frequencies of not higher than anupper threshold frequency, for example 5 kHz or 4 kHz or in a range of 2kHz with respect to the inverter WR1 and the branch Z1 associatedtherewith, respectively, this is a way of expressing in professionalterms that the power semiconductors are selected for such a switchingfrequency and that the driving is adapted and configured such that thisswitching frequency is not exceeded. A person skilled in the art maylearn the specific design and precise structure of such an inverter fromsaid functional data. For such an inverter, of course, also powersemiconductors may be used which are capable of switching higherfrequencies, it is only that such an inverter would be correspondinglyhigher in cost and its cooling requirement would be higher. From anexpert's point view, an inverter is chosen which is low in cost andequipped with semiconductors just capable of switching a thresholdfrequency and a driving is chosen which is adapted thereto and operablewith two regulators and the internal pulse patterns of WR1, WR2, asdepicted symbolically in FIG. 2.

The A-branch, which is associated with branch Z1 of FIG. 1, is adaptedfor this low switching frequency which is, however, still considerablyhigher than the nominal frequency of the machine M to be connectedthereto and which is also considerably higher than the actual frequencyof the supplied alternating currents sometimes required for operation ofthis machine, for example, in highly dynamic control operations or inmachine tools or other highly dynamic current regulations andapplications requiring fast regulating operations. Such applicationscomprise, e.g. a cross cutter (synchronized to a web speed) or printingmachines (synchronized to a printer's imprint).

The alternative feeding of the two inverters WR1, WR2, shown in FIG. 5,either via a multi-phase alternating system (e.g. a three-phase network)or a common intermediate circuit, is illustrated in FIG. 1 for theexample of the intermediate circuit having the intermediate circuitvoltage Ug, wherein all branches of these inverters are feed by thisintermediate circuit which may be common to both inverters WR1, WR2.

FIG. 1 shows the first branch of the first inverter WR1 and a branch ofthe second inverter WR2 corresponding to this phase of the alternatingsystem to be produced, in the example, the one which is supplied to themotor winding L1, L2, L3 are fed accordingly.

Here, the concept of “phase” is to be understood such that there is abranch which defines an individual unit in the load part of the inverterand which, together with another branch of the second inverter, definesa winding phase S1 which appears to be a new inverter, when viewed fromoutput terminal W1. This winding phase then feeds the first winding ofthe rotor or stator of the machine M. A phase is then analogously thealternating voltage output to W1 or the output alternating currenthaving the actual frequency f₁, which phase is combined to form athree-phase system when three correspondingly configured phases areprovided. In order to avoid confusion with this “phase”, the twobranches of FIG. 1 operating in parallel operation and associated withnon-identical frequency converters are referred to as winding phase.They are referred to as first winding phase S1, which is also associatedwith the control and driving, respectively, according to FIG. 2 forimplementing the current regulation of the two alternating currentsoutput by the two branches z1, Z1 and for superimposing said currents inthe interconnected inductor DR.

It may be learned from the function of the two component currents 1A and1B of FIG. 3 that the current portion of higher frequency, which is thesupplementary current, is able to compensate the generated currentripple of the first component current corresponding to the mainalternating current. This compensation does not have to be complete, butmay essentially result in obtaining a smoother profile of the actualvalue of the alternating current being as close to the nominal valuei_(nominal)(t) as possible so that a sum current according to FIG. 4 isobtained which is also referred to as superimposed current.

The only ripple viewed by the motor and the correspondingly highlyattenuated noise are obtained with a lesser effort than if a mainconverter of the power category of the motor was chosen which is capableof switching the high switching frequency, i.e. is required to meet bothpower limitations, the nominal current of the motor and the highswitching frequency required for low noise and high dynamics.

This comparison shows that the weight and cost of two smaller convertersare, in sum, more favorable than the weight and cost of a largeconverter having both threshold values in current and switchingfrequency. In addition, when combining two non-identical inverters,savings may be achieved, which manifest themselves by only one controlelectronics unit, only one network supply unit and a common housing. Itmay further be contemplated that optimization potentials may beexploited by a purposeful adaptation of the power part of the inverterWR1 (having the low switching frequency). Here, low-cost powersemiconductors may be purposefully employed for achieving notable costand weight advantages, while causing neither a deteriorated noisebehavior nor losses in regulating dynamics.

FIG. 6 a shows an entire period of the current profile symbolized in ashort section only in FIG. 3. It is the current i_(A)(t), which isgenerated when switched at the low frequency of the first inverter WR1and the first branch Z1, respectively. A relatively large current rippleof up to 25 A can be seen at a maximum switching frequency of approx. 2kHz, which corresponds to one of the above described embodiments. Thespecific values of the current profile are such that a machine having anominal power of 55 kW is operated at a nominal current of 100 A. Thefrequency of the fundamental wave is in an order of magnitude of 35 Hz.The maximum switching frequency of the high-power inverter WR1 may bereadily recognized. If a high-frequency alternating current i_(B)(t)having a switching frequency of 16 kHz is superimposed, the profile ofFIG. 6 b is obtained as the sum output current of one winding phase (forone terminal of the machine M) Current ripples of low frequency can nolonger be recognized; they are compensated by the regulation accordingto FIG. 2 at the high-frequency switching frequency of the secondinverter WR2 and the second branch Z1, respectively, the current ofwhich is superimposed on the main sum current of FIG. 6 a by means ofthe inductor 21.

FIG. 7 a shows a magnified view of FIG. 6 a, in which the time base isshown enlarged. FIG. 7 a shows an enlarged section of FIG. 6 aillustrating five periods between points of time 2.5 ms and 5 ms. Afrequency of the first branch Z1 of inverter WR1 of approx. 2 kHz may becalculated therefrom. If the currents of the second inverter WR2switching at a higher frequency and of its branch z1 are added and ifthey are supplied to the winding L1 together, the smoothed currentaccording to FIG. 6 b is obtained in the enlarged section according toFIG. 7 b. This current comprises only small ripples as can be seen, inparticular, when comparing point of time 10 ms to FIG. 7 a. The verticalaxes are numbered identically in all of FIGS. 6 a to 7 b, wherein FIGS.6 a, 7 a illustrate current 1A and FIGS. 6 b, 7 b illustrate currenti₁(t).

The power flow at the output of the two branches z1 and Z1 is dependentupon the adjustment of inverter WR1 (A-branch). If WR1 is adjusted suchthat it provides a current never exceeding the nominal value of thecurrent, a current flows in WR2 which always raises the total current ascompared to the current of WR1 so that the nominal value is achieved,wherein no current is fed back from WR1.

The situation is different if the current of WR1 varies about thenominal value and is thus temporarily higher than the nominal value, asshown by the ripple of FIG. 7 a. In this case, current and power aretemporarily fed back to WR2. The mean value of power is approx. zero,since power is temporarily output and temporarily input.

If WR1 is adjusted such that it always provides a too large current,even with the ripple, WR2 will always act against that and take inpower.

Since the two inverters are preferably operated with a commondirect-voltage intermediate circuit Ug according to FIG. 1, a possiblyinput power of a branch or inverter is again provided to the otherbranch or inverter via the intermediate circuit and thus output to load.This input power is thus not lost, but is only lead in a circle once(compensated by the intermediate circuit).

In another embodiment, it is not necessary to provide for dynamic speedcontrol or dynamic momentum control. The second inverter WR2 is thendriven in synchronization with the first inverter WR1. An appropriatepulse pattern may provide for elimination of the current ripples havingthe lower pulse frequency in the motor.

However, a highly dynamic regulation of the current by means of acurrent regulation which is configured as explained in FIG. 2 ispreferred. The pulse pattern is obtained automatically in the secondinverter WR2. Filling the ripple according to FIG. 3 (top function) isachieved by means of the circuit and signal routing.

In current regulators, the error caused by feeding the first inverterWR1 is predetermined as a nominal value for the second inverter andcompensated by the second inverter by the higher-frequency driving inthe actuator, wherein the more dynamic current in form of the secondcurrent component i_(B)(t) is compensated at best completely or at leastsubstantially.

1. Circuit arrangement for feeding a magnetically active andelectrically operable operating means, in particular, an electricallyoperated machine (M) having at least one rotor or stator winding (L1,L2, L3), which circuit arrangement, in at least one first winding phase(S1), comprises: (a) a first branch (Z1) of a frequency converter (WR1)adapted for and operable at a switching frequency of not higher than 5kHz for outputting a main alternating current generated at saidswitching frequency and having a substantially lower operating frequency(f₁) to a winding (L1) of the magnetically active operating means (M);(b) a second branch (z1) of another frequency converter (WR2) adaptedfor and operable at a second switching frequency of more than 5 kHz foroutputting a supplementary alternating current generated at saidswitching frequency to the same winding (L1); for superimposing, in theat least one winding (L1) of the operating means, the two alternatingcurrents (i_(A)(t), i_(B)(t)) of the two branches (Z1, z1) to form a sumcurrent during parallel operation of the first and second branches (Z1,z1) of the two non-identical converters for at least the first windingphase (S1) of the circuit arrangement.
 2. Circuit arrangement accordingto claim 1, wherein at least one further winding phase (S2) of thecircuit arrangement according to claim I is adapted for feeding a secondwinding (L2) of the electrical operating means, in particular, themachine (M).
 3. Circuit arrangement according to claim 2, wherein athird winding phase (S3) of the circuit arrangement composed of twobranches of non-identical converters is provided for feeding a thirdwinding (L3) of the electrical operating means, in particular, themachine, with a third superimposed alternating current.
 4. Circuitarrangement according to claim 2, wherein the two or three firstbranches are associated with a first frequency converter (WR1), thestructure of which is adapted for operation below 5 kHz only and thedriving of which is adapted for operation below the same frequencythreshold only.
 5. Circuit arrangement according to claim 2, wherein thetwo or three second branches are associated with a second frequencyconverter (WR2), the structure of which is adapted for operation above 5kHz and the driving of which is adapted for operation above the samefrequency threshold.
 6. Circuit arrangement according to claim 1,wherein the first and second branches each comprise at least twoswitching power semiconductors (TA1, TA2; TB1, TB2)—connected inseries—which may be fed by an intermediate circuit (U_(g)) foroutputting the alternating current (i_(A)(t), i_(B)(t)), which may begenerated by the switching, at their junction (XA, XB).
 7. Circuitarrangement according to claim 1, wherein, in one phase (S1), eachbranch (Z1, z1) operates via a respective associated output winding (D1,D2) of an inductor (DR) associated with the phase on the respectivewinding (L1) of the operating means, in particular, a rotor or statorwinding (L1, L2, L3) of a machine.
 8. Circuit arrangement according toclaim 1, wherein the switching frequencies (f_(A), f_(B)) of the twobranches differ from each other by more than a factor of 2 and wherein,in particular, a relationship of the switching frequencies of the twobranches (z1, Z1) of 7 to 9 is provided.
 9. Circuit arrangementaccording to claim 1, wherein the electrically operated machine (M) tobe driven by the circuit arrangement has a power of more than 10 kW, orwherein the circuit arrangement is adapted and operable for operatingsuch a machine (M) at a nominal power of more than 10 kW.
 10. Circuitarrangement according to claim 1, wherein the first branch (Z1) of theone frequency convener (WR1) is not operable above 4 kHz and isoperable, in particular, substantially only below 3 kHz or in a range of2 kHz.
 11. Circuit arrangement according to claim 1, wherein the secondbranch (z1) of the other frequency converter (WR2) is operable above 10kHz, in particular, above 15 kHz or in a range of 16 kHz.
 12. Circuitarrangement according to claim 1, wherein the branch (z1) of thefrequency converter operable at a higher frequency, i.e. the “secondbranch”, is adapted for supplementing a ripple generated in the currentof the “first branch” (Z1) of the other frequency converter (WR1)operable at a lower frequency to form, at least substantially, an actualvalue of the alternating current having a smoother profile, as the sumcurrent output by the respective winding phase (S1).
 13. Circuitarrangement according to claim 1, wherein the sum current composed ofmain alternating current (i_(A)) and supplementary alternating current(i_(B))—output by at least one winding phase (S1) of the circuitarrangement—comprises an effective current ripple in the range of thefrequency of the branch or frequency converter (z1, WR2) operating at ahigher switching frequency (f_(B)).
 14. Circuit arrangement according toclaim 1, wherein a pulse pattern for driving the second branch (z1) ofthe other frequency converter is adjusted such that a generated currentripple of the main alternating current output by the first branch (Z1)of the one frequency converter (WR1) is compensated at least to a largeextent or may be compensated, respectively.
 15. Circuit arrangementaccording to claim 8, wherein the switching frequencies have arelationship of 2^(n), wherein “n” is an integer greater than one. 16.Circuit arrangement according to claim 1, wherein the switchingfrequencies of the two branches differ from each other substantially bya factor of
 2. 17. Circuit arrangement according to claim 9, wherein thepower is greater than 30 kW.
 18. Circuit arrangement according to claim1, wherein an inductor (DR; 21) comprises two windings (D1, D2)magnetically coupled in the same direction and connected to each otherin an electrically conducting manner for superimposing the currentsgenerated at different frequencies (f_(A), f_(B)) i.e. the main andsupplementary alternating currents (i_(A), i_(B)), so that the sumcurrent (i₁(t)) can be output as a superimposed current at the junction(20) of the magnetically coupled windings.
 19. Circuit arrangementaccording to claim 1, wherein the main and supplementary alternatingcurrents may be fed to an inductor (DR) having a tap (20) for outputtingthe sum current for the at least one winding (L1, L2, L3) of theoperating means, in particular the electric machine (M), at the tap(20).
 20. Method for operating a circuit arrangement for feeding anelectrically operated operating means having at least one winding (L1),such as a machine (M) having at least one rotor or stator winding (L1,L2, L3), which circuit arrangement, in at least one first winding phase(S1), comprises: (a) a first branch (Z1) of a frequency converter (WR1)adapted for and operable at switching frequencies of not higher than 5kHz, wherein a main alternating current generated at said switchingfrequencies is output to the at least one winding (L1); (b) a secondbranch (z1) of another frequency converter (WR2) adapted for andoperable at switching frequencies of more than 5 kHz, wherein asupplementary alternating current generated at said switchingfrequencies is output to the same winding (L1); wherein, in the at leastone winding (L1) of the electrically operated operating means, the twoalternating currents (i_(A)(t), i_(B)(t)) of the two branches (Z1, z1)are superimposed during parallel operation of the first and secondbranches (Z1, z1) of the two non-identical frequency converters for atleast the first winding phase (S1) of the circuit arrangement. 21.(canceled) 21a. (canceled) 21b. (canceled)
 22. Driving arrangement for acircuit arrangement according to claim 1 and for regulating orcontrolling a sum current (i₁), wherein at least one current regulator(V1, V2) for the current having the higher switching frequency isprovided for at least one first winding phase (S1), wherein the at leastone current regulator (V2) for the current having the higher switchingfrequency may be supplied or is supplied with a difference between anominal current value (i_(nominal)) for a first winding phase (S1) and ameasured current (i_(A)), as a nominal value, at the output of the firstbranch (Z1) and in front of an inductor (DR, 21).
 23. Drivingarrangement according to claim 22, wherein a further regulator (V1) isprovided for the current having the lower switching frequency, and thefurther regulator (V1) for the current having the lower switchingfrequency (f_(A)) may be supplied with the nominal current value(i_(nominal)) for the first winding phase (S1) as a nominal value; andwherein each one of the two current regulators (V1, V2) discretelycontrols or regulates the output current (i_(A), i_(B)) at the branch(z1, Z1) associated therewith according to its respective predeterminednominal value.
 24. Driving arrangement according to claim 22, whereinthe two currents (i_(A), i_(B))—which are controlled or regulated by oneor both current regulators (V1, V2)—are supplied, as the sum current(i₁), to the output (W1) of the winding phase (S1) via an inductor (DR;21; 21 a, 21 b, 21 c) having a tap (20).
 25. Driving arrangement for acircuit arrangement according to claim 1, for regulating or controllinga sum current (i₁), wherein two current regulators (V1, V2) are providedfor at least one first winding phase (S1), wherein the second currentregulator (V2) for a current which may be generated at the higherswitching frequency may be supplied with a difference between a nominalcurrent value (i_(nominal)) of a first winding phase (S1) and a measuredcurrent (i_(A)), as a nominal value, at the output of the first branch(Z1) and in front of an inductor (DR, 21); the first regulator (V1) fora current which may generated at a lower switching frequency (f_(A)) maybe supplied with the nominal current value (i_(nominal)) of the firstwinding phase (S1) as a nominal value; and wherein each one of the twocurrent regulators (V1, V2) discretely controls or regulates the outputcurrent (i_(A), i_(B)) of the branch (z1, Z1) associated therewithaccording to its respective predetermined nominal value.
 26. Arrangementaccording to claim 25, wherein the two currents (i_(A), i_(B)), whichare controlled or regulated by the two current regulators (V1, V2), aresupplied, as the sum current (i₁), to the output (W1) of the firstwinding phase (S1) via the inductor (DR; 21; 21 a, 21 b, 21 c) having atap (20).
 27. Arrangement according to claim 26, wherein a currentacquisition influencing the two current regulators is not performedbehind said inductor (DR), however, the sum current of the two branches(z1, Z1) of the first winding phase (S1) is supplied or may be suppliedto one of a plurality of operating-means windings (L1).
 28. Arrangementaccording to claim 7, wherein the inductor (DR) has a continuous windingin the same direction including a tap (20).
 29. Arrangement according toclaim 25, wherein a lower switching frequency of the first branch (Z1),in particular, on average over a period of the sum current (i₁), ishigher than substantially 500 Hz or is no less than the ten-fold nominalfrequency of the sum current (i₁).
 30. Circuit arrangement for feeding amagnetically active and electrically operable operating means andincluding a driving arrangement, which circuit arrangement, in at leastone first winding phase (S1), comprises: (a) a first branch (Z1) of afrequency converter (WR1) adapted for and operable at a switchingfrequency of not higher than 5 kHz for outputting a main alternatingcurrent generated at said switching frequency and having a substantiallylower operating frequency (f₁) to a winding (L1) of the magneticallyactive operating means (M); (b) a second branch (z1) of anotherfrequency converter (WR2) adapted for and operable at a second switchingfrequency of more than 5 kHz for outputting a supplementary alternatingcurrent generated at said switching frequency to the same winding (L1);for superimposing, in the at least one winding (L1) of the operatingmeans, the two alternating currents (i_(A)(t), i_(B)(t)) of the twobranches (Z1, z1) to form a sum current during parallel operation of thefirst and second branches (Z1, z1) of the two non-identical convertersfor at least the first winding phase (S1) of the circuit arrangement,and wherein said driving arrangement comprises: (c) at least one currentregulator (V1, V2) for the current having the higher switching frequencyfor regulating or controlling the sum current (i₁) for at least thefirst winding phase (S1) and wherein the current regulator (V2) for thecurrent having the higher switching frequency may be supplied with adifference between a nominal current value (i_(nominal)) for the firstwinding phase (S1) and a measured current (i_(A)) at the output of thefirst branch (Z1).
 31. Circuit arrangement according to claim 30,wherein at least one further winding phase (S2) of the circuitarrangement according to claim 1 is adapted for feeding a second winding(L2) of the electrical operating means, in particular, a machine (M).32. Circuit arrangement according to claim 30, adapted and suitable fordriving an electrically operated machine (M) having at least one rotoror stator winding (L1, L2, L3).
 33. Circuit arrangement according toclaim 30, wherein a further regulator (V1) is provided for the currenthaving the lower switching frequency, and the further regulator (V1) forthe current which may be generated at the lower switching frequency(f_(A)) may be supplied with the nominal current value (i_(nominal)) ofthe winding phase (S1) as a nominal value so that at least two currentregulators (V1, V2) are provided; and wherein each one of the twocurrent regulators (V1, V2) discretely controls or regulates the outputcurrent (i_(A), i_(B)) at the branch (z1, Z1) associated therewithaccording to its respective predetermined nominal value specification.34. Circuit arrangement according to claim 30, wherein the two currents(i_(A), i_(B)), which are controlled or regulated by the two currentregulators (V1, V2), are supplied, as the sum current (i₁), to theoutput (W1) of the winding phase (S1) via an inductor (DR; 21; 21 a, 21b, 21 c) having a tap (20).
 35. Method for operating a circuitarrangement for feeding an electrically operated operating means havingat least one winding (L1), which circuit arrangement comprises a drivingarrangement and, in at least one first winding phase (S1): (a) a firstbranch (Z1) of a frequency converter (WR1) adapted for and operable atswitching frequencies of not higher than 5 kHz, wherein a mainalternating current (i_(A)(t)) generated at said switching frequenciesis output to the at least one winding (L1); (b) a second branch (z1) ofanother frequency converter (WR2) adapted for and operable at switchingfrequencies of more than 5 kHz, wherein a supplementary alternatingcurrent (i_(B)(t)) generated at said switching frequencies is output tothe same winding (L1); (c) wherein at least one current regulator (V1,V2) for the current generated at the higher switching frequency isprovided for regulating or controlling a sum current (i₁) for at leastthe first winding phase (S1) and wherein the current regulator (V2) forthe current generated at the higher switching frequency is supplied witha difference between a nominal current value (i_(nominal)) for the firstwinding phase (S1) and a measured current (i_(A)), as a nominal value,at the output of the first branch (Z1); wherein, in the at least onewinding (L1) of the electrically operated operating means, the twoalternating currents (i_(A)(t), i_(B)(t)) of the two branches (Z1, z1)are superimposed during parallel operation of the first and secondbranches (Z1, z1) of the two non-identical converters for at least thefirst winding phase (S1) of the circuit arrangement.
 36. Methodaccording to claim 35, wherein the upper switching frequency of thefirst branch (Z1) is at least ten times greater than the operatingfrequency or nominal frequency of the generated main alternating current(i_(A)(t)).
 37. Method according to claim 35, wherein the winding is arotor or stator winding of a machine.
 38. Method according to claim 35,wherein operability of the respective inverter or its branch defines thestructural property thereof.
 39. Method according to claim 20, wherein alower threshold frequency of the first branch (Z1), on average over aperiod of the main alternating current (i_(A)(t)), is ten times greaterthan the nominal frequency of the main alternating current.
 40. Methodaccording to claim 20, wherein the lower switching frequency of thesecond branch (z1), on average over a period of the main alternatingcurrent (i_(A)(t)), is above 10 kHz.